Gummosis is a common wound response that results in the exudation of a gum sealant at the site of cracks in bark. A. M. Stephen et al., “Exudate Gums”, Methods Plant Biochem. (1990). Generally the exudate is a composite of polysaccharides and glycoproteins structurally related to cell wall components such as galactans [G. O. Aspinall, “Plant Gums”, The Carbohydrates 2B:522536 (1970)] and hydroxyproline-rich glycoproteins [Anderson and McDougall, “The chemical characterization of the gum exudates from eight Australian Acacia species of the series Phyllodineae.” Food Hydrocolloids, 2: 329 (1988)].
Gum arabic is probably the best characterized of these exudates (although it has been largely refractory to chemical analysis). It is a natural plant exudate secreted by various species of Acacia trees. Acacia senegal accounts for approximately 80% of the production of gum arabic with Acacia seyal, Acacia laeta, Acacia camplylacantha, and Acacia drepanolobium supplying the remaining 20%. The gum is gathered by hand in Africa. It is a tedious process involving piercing and stripping the bark of the trees, then returning later to gather the dried tear drop shaped, spherical balls that form.
The exact chemical nature of gum arabic has not been elucidated. It is believed to consist of two major components, a microheterogeneous glucurono-arabinorhamnogalactan polysaccharide and a higher molecular weight hydroxyproline-rich glycoprotein. Osman et al., “Characterization of Gum Arabic Fractions Obtained By Anion-Exchange Chromatography” Phytochemistry 38:409 (1984) and Qi et al., “Gum Arabic Glycoprotein Is A Twisted Hairy Rope” Plant Physiol. 96:848 (1991). While the amino composition of the protein portion has been examined, little is known with regard to the precise amino acid sequence.
While the precise chemical nature of gum arabic is elusive, the gum is nonetheless particularly useful due to its high solubility and low viscosity compared to other gums. The FDA declared the gum to be a GRAS food additive. Consequently, it is widely used in the food industry as a thickener, emulsifier, stabilizer, surfactant, protective colloid, and flavor fixative or preservative. J. Dziezak, “A Focus on Gums” Food Technology (March 1991). It is also used extensively in the cosmetics industry.
Normally, the world production of gum arabic is over 100,000 tons per year. However, this production depends on the environmental and political stability of the region producing the gum. In the early 1970s, for example, a severe drought reduced gum production to 30,00 tons. Again in 1985, drought brought about shortages of the guru, resulting in a 600% price increase.
Three approaches have been used to deal with the somewhat precarious supply problem of gum arabic. First, other gums have been sought out in other regions of the world. Second, additives have been investigated to supplement inferior gum arabic. Third, production has been investigated in cultured cells.
The effort to find other gums in other regions of the world has met with some limited success. However, the solubility of gum arabic from Acacia is superior to other gums because it dissolves well in either hot or cold water. Moreover, while other exudates are limited to a 5% solution because of their excessive viscosity, gum arabic can be dissolved readily to make 55% solutions.
Some additives have been identified to supplement gum arabic. For example, whey proteins can be used to increase the functionality of gum arabic. A. Prakash et al., “The effects of added proteins on the functionality of gum arabic in soft drink emulsion systems,” Food Hydrocolloids 4:177 (1990). However, this approach has limitations. Only low concentrations of such additives can be used without producing off-flavors in the final food product.
Attempts to produce gum arabic in cultured Acacia senegal cells has been explored. Unfortunately, conditions have not been found which lead to the expression of gum arabic in culture. A. Mollard and J-P. Joseleau, “Acacia senegal cells cultured in suspension secrete a hydroxyproline-deficient rabinogalactan-protein” Plant Physiol. Biochem. 32:703 (1994).
Clearly, new approaches to improve gum arabic production are needed. Such approaches should not be dependent on environmental or political factors. Ideally, such approaches should simplify production and be relatively inexpensive.